Many vehicles are used over a wide range of vehicle speeds, including both forward and reverse movement. Some types of engines, however, are capable of operating efficiently only within a narrow range of speeds. Consequently, transmissions capable of efficiently transmitting power at a variety of speed ratios are frequently employed. Transmission speed ratio is the ratio of input shaft speed to output shaft speed. When the vehicle is at low speed, the transmission is usually operated at a high speed ratio such that it multiplies the engine torque for improved acceleration. At high vehicle speed, operating the transmission at a low speed ratio permits an engine speed associated with quiet, fuel efficient cruising.
A common type of automatic transmission includes a gearbox capable of alternately establishing a fixed number of power flow paths, each associated with a fixed speed ratio. The gearbox includes a number of shift elements such as clutches and brakes. A particular power flow path is established by engaging a particular subset of the shift elements. To shift from one power flow path to another power flow path with a different speed ratio, one or more shift elements must be released while one or more other shift elements must be engaged. Some shift elements are passive devices such as one way clutches, while other shift elements engage or disengage in response to commands from a controller. For example, in many automatic transmissions, the shift devices are hydraulically controlled friction clutches or brakes. The controller regulates the torque capacity of the shift element by regulating an electrical current to a solenoid, which adjusts a force on a valve which, in turn, adjusts a pressure in a hydraulic circuit.
Most transmissions are equipped with a launch device. When the vehicle is stationary or moving very slowly, the gearbox input speed is less than the minimum operating speed of the engine. A launch device transmits torque from the engine to the gearbox input while permitting the engine to rotate at an acceptable speed. A common launch device is a torque converter which includes an impeller driven by the engine and a turbine driving the gearbox input. Torque is transferred from the impeller to the turbine hydro-dynamically. Many torque converters also include a hydraulically controlled lock-up clutch that couples the impeller to the turbine, bypassing the hydro-dynamic power transfer path to improve efficiency at higher vehicle speeds. Other transmissions use an actively controlled launch clutch as a launch device.
A modern automatic transmission is controlled by a microprocessor which adjusts the torque capacity of each shift element, including any lock-up clutch, at regular intervals. At each interval, the controller gathers information indicating the driver's intent, such as the positions of the shifter (PRNDL), the accelerator pedal, and the brake pedal. The controller also gathers information about the current operating state of the vehicle, such as speed, and of the engine. Increasingly, information is also available from other sources, such as anti-lock brake controllers and GPS systems. Using this information, the controller determines whether to maintain the currently established power flow path or to shift to a different power flow path. If the controller decides to shift to a different power flow path, the controller then adjusts the torque capacities of the off-going shift elements and the on-coming shift elements in a coordinated manner in order to make the transition as smooth as possible. The controller makes these determinations using computer code called the control strategy. The control strategy often includes a large number of calibratable parameters. Before a new vehicle design is mass produced for sale to the public, engineers must set the values of these calibratable parameters. The most common process for setting these values involves driving physical prototype vehicles through a large number of scenarios, observing whether the vehicle executes each scenario in a pleasing manner, and if not, adjusting the values of relevant calibratable parameters and repeating the scenario until the result is pleasing. Scenarios include, for example, shifts from particular gear ratios to particular gear ratios at particular speeds and accelerator pedal positions with particular environmental conditions. Due to the large number of scenarios and large number of calibratable parameters, the calibration process is very time consuming and costly.